Those who believe they can function well on six or fewer
hours of sleep every night may be accumulating a "sleep
debt" that cuts into their normal cognitive abilities,
according to research conducted at the University
of Pennsylvania School of Medicine. What's more,
the research indicates, those people may be too sleep-deprived
to know it.

The study, published in the March 15 issue of the journal
Sleep, found that chronically sleep-deprived
individuals reported feeling "only slightly sleepy"
even when their performance was at its worst during
standard psychological testing. The results provide
scientific insight into the daily challenges that confront
military personnel, residents and on-call doctors and
surgeons, shift workers, parents of young children,
and others who routinely get fewer than six hours of
sleep each night.

"Routine nightly sleep for fewer than six hours
results in cognitive performance deficits, even if we
feel we have adapted to it," said Hans P.A.
Van Dongen, PhD, Assistant Professor of Sleep and
Chronobiology in Penn's Department of Psychiatry and
corresponding author of the study. "This work demonstrates
the importance of sleep as a necessity for health and
well-being. Even relatively moderate sleep restriction,
if it is sustained night after night, can seriously
impair our neurobiological functioning."

David F. Dinges, PhD, Professor of Psychology
in the Department of Psychiatry and Chief of the Division
of Sleep and Chronobiology, served as principal investigator
for the study.

Dinges, Van Dongen and their colleagues looked at the
effects of four hours nightly sleep and six hours nightly
sleep on healthy volunteer subjects aged 21 to 38, over
a two-week period. They compared the results of the
subjects' accumulating performance deficits, determined
by standard psychomotor vigilance and other cognitive
tests, with similar test results obtained from subjects
who had gone without sleep for more than three nights.

The first group of subjects experienced increasing lapses
in psychomotor vigilance over days, resulting in a decline
of performance that matched that of the subjects who
went without sleep for 88 hours. At that level, the
subjects suffered lapses in their ability to react that
would put them at risk driving or flying an airplane.
They were also less able to multi-task successfully.
"The physiologic expression of sleep in humans
appears to have multiple functions, ranging from the
metabolic to the neurocognitive," Van Dongen said.

Other scientists who worked on the study are Greg
Maislin, MS, MA, also of Penn, and Janet M. Mullington,
PhD, of Beth Israel Deaconess Medical Center and Harvard
University.

The study was funded by the National Institute of Nursing
Research of the National Institutes of Health with additional
financial assistance from the National Center for Research
Resources and the National Heart, Lung and Blood Institute.